Distributed Generators (DGs) for power generation are becoming more prevalent due to their advantages over central power generation methods. By harvesting energy from renewable energy sources, DGs can provide a sustainable solution for future power generation. In order to accommodate a high number of DGs, the utility grid should be able to tolerate relatively high fluctuations in the frequency and amplitude of the grid voltage due to the dynamics the DGs impose on the utility grid. In order to accommodate these fluctuations, the standards for grid-connected power converters have rapidly been changing. The introduction of Rule 21 of “Test Protocols for Advanced Inverter Interoperability Functions” from Sandia National Laboratories for power converters with grid interconnection is one example of these changes.
Grid-connected power converters must be synchronized with the grid voltage. Therefore, synchronization is an inevitable part of the control system for any grid-connected power converter. The grid synchronization block of the control system extracts the phase information of the grid voltage and synchronizes the power converter accordingly. It also extracts other attributes of the grid voltage such as amplitude whenever needed.
The grid synchronization technique should be able to quickly extract the various parameters of the grid voltage even while in the presence of harmonics/noise. In particular, as the number of DGs coupled to the grid increase, there will be a significant amount of variations in the grid voltage. Thus, the synchronization technique must be able to properly synchronize with the grid despite the fluctuations and harmonics/noise in the grid voltage.
Grid synchronizations based on the Adaptive Notch Filter (ANF) PLL and on the Second-Order Generalized Integrator (SOGI) PLL are the current state-of-the-art techniques used for grid synchronization. Both the ANF-based PLL and SOGI-based PLL each utilize two integrators in order to generate orthogonal signals. These orthogonal signals are then used to extract the phase/frequency information. Although these techniques provide a simple and practical solution for grid synchronization, they are susceptible to the harmonic content of the grid voltage. In order to mitigate the impact of harmonics, multiple modules need to be implemented for different harmonics.
FIG. 1 shows the block diagram of the ANF-based PLL (Prior Art) while FIG. 2 shows the block diagram of the SOGI-based PLL (Prior Art). According to FIG. 1 and FIG. 2, both the ANF-based PLL and the SOGI-based PLL use two integrators (i.e., second order generalized integrator) to produce the orthogonal signals. Therefore, in terms of producing orthogonal signals, both methods have the same structure. However, the phase extraction technique from the orthogonal signals is different for the ANF-based PLL and the SOGI-based PLL. For the SOGI-based PLL, phase extraction is based on the dq-transformation (Park transformation) and PI controller for the SOGI-based PLL. For the ANF-based PLL, phase extraction is based on an adaptive algorithm.
Neither the ANF-based PLL nor the SOGI-based PLL is able to precisely extract the attributes of the grid voltage in the presence of harmonics/noise and fluctuations. Therefore, grid-connected power converters require a more effective solution to be able to integrate into future DGs.
Based on the above, there is therefore a need for systems and devices which mitigate if not avoid the shortcomings of the prior art.